This invention relates to polycarbonates, and more particularly to a method for their preparation by solid state polymerization.
Solid state polymerization (hereinafter sometimes "SSP") of polycarbonates is disclosed, for example, in U.S. Pat. No. 4,948,871, 5,204,377 and 5,214,073, the disclosures of which are incorporated herein. It involves three steps: a first step of forming a prepolymer, typically by melt polymerization (i.e., transesterification) of a dihydroxyaromatic compound such as bisphenol A with a diaryl carbonate such as diphenyl carbonate; a second step of crystallizing the prepolymer; and a third step of building the molecular weight of the crystallized prepolymer by heating to a temperature between its glass transition temperature (Tg) and its melting temperature. Use of this polymerization method is of increasing interest by reason of its effectiveness and environmental benefits.
The second or crystallization step of this method is performed, according to said patents, by solvent treatment or heat treatment. As described, the solvent treatment method may in fact employ a good or poor solvent for the prepolymer, with contact involving either the liquid or vapor form thereof. Illustrative "solvents" include aliphatic aromatic hydrocarbons, ethers, esters, ketones and halogenated aliphatic and aromatic hydrocarbons.
More recent teachings have described SSP methods that require only surface crystallization of the precursor polycarbonate particles. For example, according to copending, commonly owned application Serial No. 08/986,448 surface crystallization is effected by contact, typically at a temperature in the range of about 20-50.degree. C., with at least one dialkyl carbonate, optionally in the presence of another non-solvent such as water or a C.sub.1-4 alkanol. Alternatively, copending, commonly owned application Serial No. [RD-26032] describes a crystallization method in which contact is with water or a C.sub.1-20 alkanol in the liquid or vapor state, within a specifically defined temperature range which begins well below the boiling point of the water or alkanol.